Apply texture to mesh that has been extruded
If you make a change to your mesh you will need to UV Unwrap it again.

In Object Mode Apply the Scale Ctrl + A > Scale. 

Also, you could use the Project From View method of UV Unwrapping since it's flat. Just make sure your looking at it straight on.

A quick tip on Modeling the wall: Unless you plan on extruding "out" geometry your mesh would be much "cleaner" if, in Edit Mode,  you just selected the bottom two vertices (or top two) and pressed G + Z to grab them and move the only on the Z Axis. Then your left with a simple rectangle. If you Extrude the mesh down you are creating extra vertices and in complicated models that will increase render times unnecessarily.